The oil and gas industry has been expanding its exploration and production operations from land to sea since the 1890s. The first submerged oil well was drilled in fresh waters in Ohio in 1891. In 1897, the first derrick was placed atop a wharf about 250 feet from the California shoreline. However, true offshore drilling and production did not take off until the first well was drilled completely out of site of land in 1947. Since then, advancing technology has allowed for the drilling of wells and recovery of oil and gas at greater water depths.
Underwater oil fields are generally split into shallow water and deep-water categories because different equipment and approaches are used for oil recovery. Shallow water drilling and recovery occurs at depths less than 500 feet and generally involve rigs that have legs long enough to reach the bottom of the sea floor.
As drilling extends further offshore, rigs have become larger and more complex to meet the hostile environment. Furthermore, time to perform operations are much greater in deep-water than shallow water operations. Thus, deep-water drilling has been economically infeasible in the past. But, rising oil prices and depleted shallow water fields are making deep-water drilling more and more attractive.
FIG. 1 displays the many types of deep-water production systems in use today. For deep-water drilling (>500 feet), semi-submersible drilling rigs and drillships have traditionally been used and use of other floating production systems are increasing. However, for huge water depths, these methods are not very cost effective. The ocean can add several hundred meters or more to the fluid column, which increases the equivalent circulating density and downhole pressures in drilling wells, as well as the energy needed to lift produced fluids for separation on the platform.
Subsea facilities that reside on the sea floor, are being increasingly used as they become being more economical and technically feasible for use at great water depths. Here, the subsea equipment is attached to a platform anchored to the sea floor and a flowline conducting the produced hydrocarbons is connected to another structure, such as a floating tanker.
With the equipment being located on the sea floor, subsea systems provide for a less expensive solution for a myriad of harsh conditions than is provided by other technology. Furthermore, the requirement for the system to work deep underwater potentially reduces oil spills because connections must be sealed to prevent water ingress. Equipment working at atmospheric pressure may not meet such design requirements. Thus, subsea installations can help to exploit resources at progressively deeper waters, at locations that have previously been inaccessible, and at locations with harsh environmental conditions, such as the Barents Sea where drifting sea ice can damage surface equipment.
Improvements in technology have allowed subsea facilities to perform numerous processes that have traditionally occurred at the surface, thus debottlenecking the processing capacity. For example, some newer processes being performed by subsea facilities include water removal and re-injection or disposal, single-phase and multi-phase boosting of well fluids, sand and solid separations, and gas/liquid separations. This reduces the need for flowlines and risers that lift these components from the subsea facility for separation and then return them back to the seafloor for re-injection.
However, a disadvantage of subsea production systems is the cost of installation and maintenance of subsea equipment. First, a platform has to be installed on the sea floor. Accurate positioning requires time and skill and installation can be affected by bad weather on the surface.
Once a platform is installed, the subsea equipment can be attached to the platform. Generally, this includes a wellhead, valve tree equipment, pipelines, structures and piping systems, and the like. The installation and maintenance of subsea equipment requires specialized and expensive methods, including regular diving equipment for shallow work up to 300 meters; one atmosphere diving equipment for work up to 700 meters; robotic equipment, generally remotely operated underwater vehicles (ROVs), for deeper depths; and, specialized ships equipped with large cranes to lower and raise equipment.
Subsea equipment installation is currently performed by first lowering the equipment using a large crane. Ship positioning and crane manipulation are paramount to accurately placing equipment. Affecting these methods is the length of cable guidelines needed for deeper waters wherein longer cables increase the effects of pendulum-like oscillations. Also, the weight and size of each load is limited by the crane's capacity (including the weight of the crane wire) and the crane's reach. Thus, larger facilities have to be broken down into many pieces to prevent overloading of the crane, resulting in even more bottom trips being made.
Once the equipment is near the sea floor, remote controlled vehicles (ROVs) are used to maneuver the equipment into the desired location before landing and to connect each component to the platform and to the subsea system.
Once all of the pieces are unloaded and installed on the subsea facility, the system is then tested for functionality. If there is a problem with system performance, then the troubled component(s) must be disconnected and brought up to the surface for repair or to be exchanged.
Therefore it is readily apparent that complete subsea installation procedure can be a lengthy and expensive process. Not every harbor has the specialty ships equipped with large cranes, which adds the cost of coordinating the special equipment and getting it to the deep-water site. Weather conditions limit installations due to potential damage from accelerating and decelerating forces during equipment pick-up and landing. This is especially important because many items being installed are delicate and can be damage by wave action. Lastly, simple repairs of the subsea facility can also be very expensive and time consuming if the ROVs are unable to complete the repairs underwater.
Because of these difficulties, much research in the realm of subsea oil and gas recovery focuses on methods of reducing installation cost and time. U.S. Pat. No. 4,909,671, for example, describes a method for installing a floatable or buoyant body on the sea floor. The body has a ballast system wherein the supply of ballast water is adjusted to allow the body to sink at a pre-selected velocity to a predetermined buoyancy neutral level, wherein the body is further lowered to the sea floor using a guide wire attached to a ship. As such, large structures can be lowered without the use of a conventional crane ship and independent of weather and climatic conditions. However, this patent does not address the installation of subsea equipment upon the platform, and equipment is still installed piece-by-piece.
Following the same trend, U.S. Pat. No. 6,752,100 describes a method of deploying and installing subsea equipment, particularly smaller, delicate components, using buoys instead of a ship equipped with a crane. The use of buoys essentially reduces any effect from vessel motion that may affect the accuracy of positioning the equipment. US20110164926 also uses a buoyance system for lowering equipment, however, a control weight is used to overcome the positive buoyancy of the system. In both inventions, the use of a buoyance system allows for a gentler landing of the delicate equipment. However, neither addresses the complete subsea facility installment.
Of additional concern is the cost and time needed for performing intervention or maintenance operations on a subsea facility. US20010240303 describes a subsea intervention module with a buoyance system and a navigational control that allows for the module to be lowered via a guide wire attached to a ship to the subsea well head without hitting the sea floor or the well head with force. Thus, ships specially equipped with large cranes and ROVs are not necessary for docking the intervention module. Furthermore, the intervention operation can be controlled on the surface, thus eliminating the need to launch a ROV.
The art to date, however, focuses on cost reduction to single elements of the entire installation procedure. Thus, what is needed in the art is a complete subsea facility that can be installed with minimal cost at each installation step. Specifically, what is needed in the art is a subsea facility that can be installed and maintained without help or with reduced help from cranes, rigs and ROVs. Furthermore, the installation must be able to occur independent of weather conditions and must be safe for delicate and smaller items.